We have found that the oxidative probe hydroethidine (HE) is an efficient O-1(2) quencher with k(q) = 1.1 x 10(9) M (1) s (1) in CH3CN. The structurally related MitoSOX Red had k(q) = 2.1 x 10(9) M (1) s (1) in D2O-CH3CN containing DNA. Ethidium cation, the main oxidation product, and three model compounds, 5-ethyl-5,6-dihydro-3,8-dinitro-6-phenyl-6-phenanthridinol, 3-amino-2-naphthenol, and 2-aminophenol, were lesser O-1(2) quenchers. Another product, 2-OH-ethidium cation, quenched O-1(2) with k(q) = 5.5 x 10(8) M (1) s (1) in a D2O-CH3CN mixture. Interestingly, both ethidium and 2-OH-ethidium cations are weak O-1(2) photosensitizers with quantum yields of O-1(2) production in CH3CN of 0.06 and 0.03, respectively. Thus, O-1(2) can be involved in photochemical experiments with MitoSOX Red and HE. We have also developed a simple and practical method for measuring an index of photosensitization for an individual photosensitizer in order to compare the potential for harmful effects from different household and office light sources.